This invention relates to coaxial cables such as are used for carrying high frequency electromagnetic signals, including radio, television, and microwave communications. More particularly, the invention relates to small diameter coaxial cables having improved flexibility and electrical performance relative to conventional coaxial cables.
Coaxial cables are generally of two types. Each has an inner conductor, surrounded by an outer (i.e., coaxial) conductor, with the space between the inner conductor and the outer conductor being filled with air, or a dielectric material, either a solid dielectric, or a foam dielectric. While the cables filled with air are the most effective in preventing signal loss, the space left between the inner conductor and outer conductor must be kept dry in order to avoid loss of electrical performance caused by intrusion of moisture. This often requires that the annular space be pressurized with dry air, which requires additional expensive facilities to provide dry air on a continuous basis. Cables which use a solid polymer dielectric are less expensive, but they are less efficient since air is a superior dielectric. Foam dielectrics have been widely used for many years. They provide good performance at lower cost than cables, which require that dry air be supplied to the annular space, and they are more efficient than cables, which employ solid dielectrics. It is not necessary to monitor the space between the inner conductor and the outer conductor, although moisture intrusion may be a problem if there should be a leak in the outer covering or the outer conductor.
The assignee of the present invention has obtained patents which discuss the advantages of foam dielectric filled coaxial cables and the methods by which they are made. Such coaxial cables typically have corrugated outer conductors which provide flexibility to the cables and which also resist the forces caused by differential thermal expansion between the inner conductor and outer conductor. The outer conductor is particularly subject to atmospheric conditions and may expand or contract depending on the air temperature and solar radiation. The inner conductor is subject to heating depending on the electromagnetic energy passing through it. These patents include U.S. Pat. No. 3,173,990 in which Lamons discusses the advantages of corrugating the outer conductor so that the foam dielectric is compressed at the root of the corrugations so that, in effect, each undulation compensates for differential thermal expansion independently of the others. Moisture intrusion is inhibited by the application of a viscous sealant in U.S. Pat. No. 3,394,400. Improved bending life of such cables is shown in U.S. Pat. No. 3,582,536 to be obtainable by using specific dimensions of the corrugations and metal thickness. An apparatus for carrying out annular corrugation of the outer conductor in a continuous process is disclosed in U.S. Pat. No. 3,780,556. Application of a foamed fluorocarbon resin to a corrugated coaxial cable is described in U.S. Pat. No. 4,304,713.
Coaxial cables which employ foam dielectrics between the inner conductor, typically a solid wire, and the corrugated outer conductor, usually a thin walled tube which has been corrugated after being wrapped around the dielectric foam, are widely and successfully used. Heretofore, such cables have been limited to external diameters larger than about 0.25 inch (6.35 mm). For smaller diameters, braided metal outer conductors have been used, to which hot molten tin is applied to provide a continuous metal surface for the outer conductor. These cables are not as efficient as cables with continuous tubes as outer conductors. Since typical polyethylene foam dielectric materials will not withstand the temperatures required for applying molten tin, it is necessary to use fluorocarbon dielectric materials which can withstand the temperatures required. Such materials are expensive and the cables have been found to lose efficiency resulting from leakage of the electromagnetic energy passing through the cable at frequencies greater than 1 GHz.
Alternatively, smooth wall outer cables have been used. These cables provide better electrical characteristics over the tinned braid cables. Smooth wall outer cables, however, are greatly affected by forces from differential thermal expansion in the inner and outer conductors. Also, smooth wall outer conductors can be easily crushed when in use. A 0.006 inch thick smooth copper tube having an outer diameter of 0.140 inch could be compressed by 0.030 inch by applying just over 20 lbs/in.
Therefore, it is an object of this invention to provide a small diameter (less than 0.25 inch) corrugated coaxial cable that includes a foam dielectric with an ability to resist the forces caused by differential thermal expansion between the inner and outer conductors.
It is a further object of the invention to provide a small diameter (less than 0.25 inch) corrugated coaxial cable that provides the shielding properties of a smooth wall cable.